Field of the Invention
This invention relates to novel bismuth-strontium-calcium-copper oxide compositions which are superconducting and to a process for making them.
References
Bednorz and Muller, Z. Phys. B64, 189 (1986), disclose a superconducting phase in the La-Ba-Cu-O system with a superconducting transition temperature of about 35 K. This disclosure was subsequently confirmed by a number of investigators [see, for example, Rao and Ganguly, Current Science, 56, 47 (1987), Chu et al., Science 235, 567 (1987), Chu et al., Phys. Rev. Lett. 58, 405 (1987), Cava et al., Phys. Rev. Lett. 58, 408 (1987), Bednorz et al., Europhys. Lett. 3, 379 (1987)]. The superconducting phase has been identified as the composition La1−x(Ba, Sr, Ca)xCuO4−y with the tetragonal K2NiF4-type structure and with x typically about 0.15 and y indicating oxygen vacancies.
Wu et al., Phys. Rev. Lett. 58, 908 (1987), disclose a superconducting phase in the Y-Ba-Cu-O system with a superconducting transition temperature of about 90 K. Cava et al., Phys. Rev. Lett. 58, 1676 (1987), have identified this superconducting Y-Ba-Cu-O phase to be orthorhombic, distorted, oxygen-deficient perovskite YBa2 Cu3O9−δwhere δ is about 2.1 and present the powder x-ray diffraction pattern and lattice parameters.
C. Michel et al., Z. Phys. B-Condensed Matter 68, 417 (1987), disclose the introduction of Bi into the superconductor La2−xSrxCuO4−y, to form the oxides La2−xBix, Srx−x, CuO4−y. The study was limited to compositions corresponding to the range where superconductivity was mainly observed, x−x′=0.1−0.2. Single phases were obtained when x≦3 and x′≦2. One sample of composition La1.7 Bi0.1 Sr0.2 CuO4−y has a superconducting transition temperature of about 42 K as determined from resistivity measurements as compared with about 38 K for La1.8 Sr0.2 CuO4−y.
C. Michel et al., Z. Phys. B-Condensed Matter 68, 421 (1987), disclose a novel family of superconducting oxides in the Bi-Sr-Cu-O system with composition close to Bi2Sr2Cu2O7+6. A pure phase was isolated for the composition Bi2Sr2Cu2O7+6. The X-ray diffraction pattern for this material exhibits some similarity with that of perovskite and the electron diffraction, pattern shows the perovskite subcell with the orthorhombic cell parameters of a=5.32 A (0.532 nm), b=26.6 A (2.66 nm) and c=48.8 A (4.88 nm). The material made from ultrapure oxides has a superconducting transition with a midpoint of 22 K as determined from resistivity measurements and zero resistance below 14 K. The material made from commercial grade oxides has a superconducting transition with a midpoint of 7 K.